FIG. 26 shows a conventional motor-driven power steering control system disclosed, for instance, in Japanese Unexamined Patent Application Publication No. 91240/1996 (JP-A-8-91240). The conventional system is constituted by a torque sensor 3 for detecting a steering torque, an electric motor 8 for generating a steering assist force to a steering mechanism on the basis of the steering torque detection value outputted from the torque sensor 3, an ignition key 14 for performing activating operation for the motor 8, a battery 11 for supplying a driving electric power to the motor 8 in response to operation of the ignition key 14, a vehicle speed sensor 7 for detecting a vehicle speed, and an electronic control circuit 13 for controlling the output power of the motor 8 on the basis of the steering torque detection value and the vehicle speed detection value.
The electronic control circuit 13 includes a phase compensator 21, a current command arithmetic unit 22, a comparator 23, a differential compensator 24, a proportional operation unit 25, an integral operation unit 26, an adder 27, an IG (ignition) key-on detector 31, a fault detector 32, a fail-safe processor 33, a fail relay 34, a motor driving circuit 41 and a motor current detecting circuit 42.
In the conventional system, a motor current command value I is supplied to a feedback control circuit incorporated in the electronic control circuit 13 for a predetermined time T when the ignition key 14 is turned on, allowing a motor current to flow.
At a time point when a predetermined time T0 (T0&lt;T) has lapsed, a motor current detection value i detected by the motor current detecting circuit 42 is inputted to the comparator 23 constituting a part of the feedback control circuit.
The fault detector 32 compares a predicted value Ds of duty ratio of a PWM signal as predicted on the basis of the motor current command value I with an actually measured value of the duty ratio of the PWM signal outputted from the adder 27 incorporated in the feedback control circuit to which the motor current detection value i is fed back.
When the absolute value of a difference resulting from the comparison is greater than a predetermined permissible value .DELTA.D, decision is made that the motor current detecting circuit 42 suffering a fault, whereupon a fail-safe processing is carried out.
Further, as other known technique, there may be mentioned a control system which is implemented such that a desired motor current and a motor current detection value detected by a motor current detecting means are compared with each other even in the course of ordinary control, wherein when error between the desired value and the detection value mentioned above is equal to or greater than a predetermined value, decision is made that abnormality occurs in the motor current detecting circuit, whereupon a fail-safe processing is carried out.
However, the fault detecting method for the motor current detecting circuit known heretofore is carried out as an initial check immediately after the ignition key 14 is turned on. Consequently, when a fault occurs in the motor current detecting circuit during the ordinary control, it is impossible to detect the fault, giving rise to a problem.
Furthermore, with the fault detecting method described above as the prior art technique, it is impossible to perform a normal motor current control during a period from a time point at which the fault occurred to a time point for carrying out the fail-safe processing, as a result of which an abnormal current flows through the motor during the above-mentioned period.
In particular, when such a fault occurs that the motor current detection value becomes zero, an overcurrent will then flow through the motor, which will result in generation of excessively large assist torque. On the other hand, when the fault detection period is set so short that influence of the excessively large assist torque can be neglected, then the possibility of erroneous detection becomes higher, which is unpreferable from the view point of system performance.
Accordingly, with the present invention, it is contemplated to make it possible to detect abnormality of the motor current during the ordinary control while evading flow of an abnormal current through the motor and additionally make it possible to continue the control of the motor output power even in the state where the motor current detecting circuit suffers a fault.